a) Field of the Invention
Lasers comprise an active element which is pumped by a radiation source generally by means of coupling optics and a resonator. The resonator makes laser operation possible by means of optically coupling back through a partially transparent or partially reflecting output coupling element and a highly reflective resonator element.
b) Description of the Related Art
Solid-state lasers are distinguished over other lasers particularly by the large number of possible modes of operation. For example, it is possible to realize the following modes of operation through the choice of a suitable resonator arrangement:
1. continuous-wave operation or pulsed laser operation; PA1 2. transverse basic-mode operation with approximately diffraction-limited beam or operation at reduced radiation intensity; PA1 3. coupling out of the fundamental radiation in the infrared spectral region or frequency conversion by means of generating higher harmonics in the visible or ultraviolet spectral range or by means of other optically nonlinear frequency conversion methods (combination laser); PA1 4. oscillation at various laser wavelengths for media emitting on many discrete lines as well as for continuously tunable vibronic solid-state lasers such as, e.g., titanium:sapphire lasers with continuous spectra (multiwavelength lasers); PA1 5. cascade lasers in which the solid-state laser emission pumps a second crystal; PA1 6. one or more active elements in a resonator.
There are a great many applications in which different modes of operation are required alternately or simultaneously. For example, alternating or simultaneous laser radiation of different wavelength is required in laser surgery for cutting and coagulating tissue or in laser materials processing.
In order to realize different laser radiation in one device, a technically trivial solution would be to accommodate a plurality of lasers within the device housing (e.g., DE 28 09 007 A1). However, the number of lasers is strictly limited by the proportionate increase in the size, weight and cost of the device. If the desired laser radiation is simply a matter of radiation of different wavelengths which is achievable in theory with the same active element, this laser radiation can be generated with only one laser, as is known from the prior art. In that case, the laser comprises, in addition to the active element, a plurality of resonators, each of which reflects one of the desired wavelengths.
In all of the solutions known from the prior art, the position of the optical axis for the resonator construction is given by the geometry of the active element (e.g., rod-shaped). This means particularly that if there are a plurality of resonators they must be arranged collinearly and, as a consequence, resonator elements must be moved or a loss in effectiveness will occur through the use of prisms.
DE 37 13 635 A1 describes a laser for two wavelengths comprising an active element and two resonators which are formed from the same semitransparent exit mirror and a mirror having different reflection characteristics. The elements of the laser are arranged one behind the other on the optical axis determined by the active element as follows: semitransparent mirror, active element, first mirror, shutter, second mirror. The first mirror reflects the radiation of a first wavelength and lets pass the radiation of a second wavelength, while the second mirror reflects the radiation of the second wavelength. Alternatively, this laser can emit only laser radiation of either the first wavelength or the second wavelength by means of the shutter.
DE 35 00 900 A1 also describes a laser which can emit two different wavelengths alternatively. In this case, a gas cell arranged in the resonator space is either evacuated or filled with a suitable gas so that it acts as a spectrum-selective absorber. With the gas cell evacuated, the laser emits at 10.6 .mu.m and is suitable for soft-tissue surgery. When the gas cell is filled with a suitable gas, the otherwise more intensive laser line of 10.6 .mu.m is suppressed and the laser emits at 9.6 .mu.m so that it is suitable for hard-tissue surgery.
The possibility of emission with two different wavelengths is provided in the laser according to DE 37 30 563 C1, e.g., by means of a prism inside the resonator chamber, which, by means of the different refraction of two emission wavelengths of the active element, deflects the respective radiation components onto different laser mirrors. The resonators can operate alternately or simultaneously due to these switches arranged in front of the laser mirrors.
The multiwavelength laser oscillator with geometrically coupled resonators according to DE 41 10 189 can emit laser radiation of more than two wavelengths. In this case, a polygonal prism with parallel side surfaces is arranged in the resonator space and, through rotation, puts the individual resonators into operation, each individual resonator being formed by the same output coupling unit and a special dispersive element.
All of the described solutions have in common that the resonator axes of the individual resonators of a laser are the same as the optical axis given by the active element. In order to put different resonators into operation, the radiation is either divided into its components of different wavelength by stationary elements such as filters and prisms or is deflected by moving elements onto highly reflective resonator elements having different reflection characteristics.
The first basic solution mentioned above causes a loss of effectiveness, while the second solution in particular sets strict requirements for adherence to the spatial and angular accuracy of the moving elements.
Further, the compulsory collinear arrangement of the resonators in the region of the active element allows only a very limited number of resonators.
All of the solutions known from the prior art have the decisive disadvantage that the resonators are not constructed so as to be independent from one another; rather, all of them use the same output coupling element. The laser therefore emits all of the radiation in one direction.